Extended area lighting devices that use a light guide to spread light from discrete edge-mounted CCFL or LED light sources over the extended area of the light guide are known. Edge-lit backlights used in liquid crystal displays (LCDs) are a major example of such lighting devices. Ordinarily, it is important for such lighting devices to have a color and brightness that are uniform, or at least slowly varying, as a function of position on the extended area output surface. It is also ordinarily important for such lighting devices to emit light of a substantially white color, so that the filtering action of the liquid crystal panel can produce full color pixels and pictures ranging from blue through red.
In order to extract guided-mode light out of the light guide, edge-lit backlights often configure a major surface of the light guide to have a printed pattern of diffusive paint or other scattering material, or to have a structured surface e.g. as provided by a series of grooves or prisms whose facets are designed to change the direction of light by refraction or reflection. It is not common to extract guided-mode light out of the light guide using diffractive grooves or prisms on the major surface, because diffraction has a strong wavelength dependence which could easily produce a highly colored appearance, and a highly colored appearance is unacceptable in most end-use applications.